(a) Field of the Invention
The present invention relates to a positive active material for lithium (Li) secondary battery and a method of preparing the same, and more particularly, to a positive active material for lithium secondary battery, the surface properties of which have been transformed by surface-treating a positive material of LiaNi1-x-yCoxMyO2, LiaNi1-x-yCoxMyO2-z or LiaNi1-x-yCoxMyO2-zSz with a metal alkoxide solution, and a method of preparing the same.
(b) Description of the Related Art
Due to advancing appliances miniaturization, weight reduction, and increased functionality of cordless portable appliances such as video cameras, personal phones, and personal computers, there are escalating requirements for the sources of electric power used to driving these appliances. Particularly, there have been advances in developing and studying rechargeable lithium secondary battery around the world, anticipating the need for a battery with a high energy density.
A lithium secondary battery uses as an anode and a cathode materials which can intercalate and deintercalate lithium ions, and is prepared by filling organic or polymer electrolyte between the cathode and the anode in order to move the lithium ions. The battery generates electric energy by a redox reaction when lithium ions intercalate and deintercalate in the cathode and in the anode.
Lithium secondary batteries use carbon materials or lithium metals as an anode and intercalatable/deintercalatable chalcogenide compounds as a cathode. Carbon materials are substituted for lithium metals because the latter, when used as an anode, has the disadvantage of educing dendrites with the associated danger of explosion and reduction of the recharging efficiency.
On the other hand, complex metal oxide such as LiCoO2, LiMn2O4, LiNi1-xCoxO2 (0 less than X less than 1), and LiMnO2 are now being studied for a cathode use because chrome oxide, MnO2, etc. that were initially used have problems with low recharge efficiency and safety.
Lithium secondary batteries using nickelic positive active materials have the potential to make high capacity batteries due to their high discharge capacity, but more development of nickelic active materials is needed to overcome defects associated with their low durability performance and the structural instability of LiNi1-xCoxO2 (0 less than x less than 1).
Synthesizing methods employing solid state processes, co-precipitation methods, polymer chelating agents, etc, have been developed and researched thus far on LiNi1-xMxO2 (0 less than x less than 1) powder with some Ni substituted with Co, Mn, etc, for improving structural safety features, discharge capacities, and life span properties of the basic nickel based cathode compound, LiNiO2.
LiNiO2 has disadvantages in that it is difficult to synthesize and is not practical to use in a battery because of poor durability, in spite of its having a recharge capacity of 200 mAh/g.
There is also a problem with LiCoO2 in that it has low stability at high current discharges. However, LiCoO2 has been used widely due to its excellent electrode properties, high battery voltage, and a conductance ranging from 10xe2x88x922 to 1 S/cm at room temperature.
To overcome these defects, Korea patent application No. 97-56444 recently disclosed LiNi1-xMxO2 powder with Co or Mn substituting some Ni, and LiNi1-x-yCoxMyO2 (where M=Al, Mg, Sr, La, Ce, etc.: 0 less than x less than 1, 0 less than y less than 1) powder which added a little bit of a third non-Cobalt metal. Also, U.S. Pat. No. 5,773,168 discloses an active material replaces F for some oxygen in LiNiO2.
However, there still are difficulties with the structural stability and low durability performance, despite inventions addressing the problems of the conventional art.
It is an object of the present invention to provide a positive active material for a Li secondary battery, wherein LiaNi1-x-yCoxMyO2 LiaNi1-x-yCoxMyO2. zFz, and LiaNi1-x-yCoxMyO2-zSz (where M is a metal selected from the group consisting of Al, Mg, Sr, La, Ce, V, and Ti and wherein 0xe2x89xa6xxe2x89xa60.99, 0.01xe2x89xa6y less than 0.1, 0.01xe2x89xa6zxe2x89xa60.1 and 1.00xe2x89xa6axe2x89xa61.1) powders are synthesized and then coated with a metal oxide by using a metal alkoxide solution to improve the durability, capacity, and structural stability of the battery by transforming surface structures, since surface physical properties are the most important factor affecting an electrical chemical reaction by coating the surface using a metal alkoxide solution.
It is another object to provide a method of preparation of a coated positive active material for a Li secondary battery by using a metal alkoxide solution.
In order to achieve these other objects, the present invention provides positive active materials for Li secondary battery that are coated by using a metal alkoxide solution with materials selected from the group consisting of the following:
where M is a metal selected from the group consisting of Al, Mg, Sr, La, Ce, V, and Ti and wherein 0xe2x89xa6xxe2x89xa60.99, 0.01xe2x89xa6yxe2x89xa60.1, 0.01xe2x89xa6zxe2x89xa60.1 and 1.00xe2x89xa6axe2x89xa61.1.
Also, the present invention further provides a method for preparation of the positive active material selected from the group consisting of the formulae 1 to 3.
The method comprises a step of synthesizing Ni1-x-yCoxMy(OH)2 by a coprecipitation method; a step of mixing the material with LiOH, LiF, or NaS powder; a step of producing the positive active compound of the formulae 1 to 3 by heating and cooling the mixture; and a step of coating the compound by using a metal alkoxide solution.